Gilmore attended South Pointe High School in Rock Hill, South Carolina, where he was a two-way player as quarterback and defensive back, and helped his team to a perfect 15-0 record and the South Carolina AAAA Division II title as a senior. Among his teammates were DeVonte Holloman and Jadeveon Clowney. Playing quarterback on offense, Gilmore rushed for 1,331 yards and 23 touchdowns and passed for 1,771 yards with 14 scores. He earned first-team all-state honors by The State and was named an All-American by Parade and EA Sports. He was also named Mr. Football for the state of South Carolina.
Considered a four-star recruit by Rivals.com, Gilmore was listed as the No. 2 overall prospect from South Carolina in the class of 2009. He chose South Carolina over Alabama, Tennessee, and Clemson.
Gilmore graduated from South Pointe in December 2008 to enroll at South Carolina early, and participate in spring practice. It worked out, as he came out of spring as a starter at cornerback. Appearing in all 12 games for the Gamecocks, Gilmore was the rare true freshman to start at cornerback in the Southeastern Conference in 2009. He had 52 tackles, five tackles for loss, two sacks, and nine passes defended, which earned him Freshman All-American honors by College Football News and Phil Steele. He has occasionally appeared on offense, with a career total of 2 attempted passes (1 complete) and 6 rushes during regular season games. He also completed a 29 yard pass to Alshon Jeffery in the 4th quarter of the 2010 Chick-fil-A Bowl.
On May 17, 2012, Gilmore signed a rookie contract reported to be for four years (including an option for a fifth year) for $12.1 million dollars.
Upon joining the team, Gilmore was penciled in as the teams' number one cornerback and routinely covered the top receivers of opposing teams. Over the course of his rookie season, Gilmore totaled 61 tackles and one interception, coming against the St. Louis Rams on December 9, 2012.